An UWB communication system is defined as a communication system having a bandwidth of 25% or more of a center frequency, or 1.5 GHz or more. UWB communication employs a signal whose power is diffused over a wide frequency band, such as an impulse signal. That is, a pulse having a several nanosecond to picosecond width (duration) is used in order to diffuse power over a wide frequency band of a GHz order. The UWB communication scheme is a communication scheme having a bandwidth much wider than that of a wideband CDMA communication scheme having a bandwidth of about 5 MHz.
In the UWB communication system, a signal is modulated so as to transfer information using a short pulse. A modulation method, such as OOK (On-Off Keying), PAM (Pulse Amplitude Modulation) or PPM (Pulse Position Modulation), is used in order to modulate a signal while maintaining a wideband characteristic of a pulse itself. Therefore, the UWB system is simple in structure and easy in implementation since it does not require a carrier. Furthermore, since power is diffused over a wide band, each frequency component requires very low power. This makes the UWB system less interfere with other communication systems that employ a narrow frequency band and also makes wiretapping difficult. Accordingly, the UWB system is suitable to maintain communication security. Furthermore, the UWB system is advantageous in that it allows for high-speed communication with very low power and has a good obstacle transmittance characteristic.
Due to the advantages, it is expected that the UWB system will be widely used in the field of the next-generation Wireless Personal Area Network (WPAN), such as a wireless home network. More particularly, U.S. Federal Communications Commission (FCC) approved that the UWB communication method could be used commercially at a frequency band of 3.1 GHz or more on February 2002. This accelerates the commercialization of the UWB system.
The UWB system employs a wide frequency band in comparison with a conventional communication system. It is therefore inevitable to develop a small antenna having a wideband characteristic suitable for the wide frequency band. An antenna for the UWB system generally includes a horn antenna, a bi-conical antenna, and so on. U.S. Pat. No. 6,621,462 issued to Time Domain Corporation, U.S. Pat. No. 6,590,545 issued to Xtreme Spectrum, Inc., etc. disclose other types of UWB antennas.
However, these antennas are problematic in that they are inappropriate for the fields requiring small and lightweight antennas because of its size.
Korean Patent Application No. 2003-49755 assigned to LG Electronics, Co., Ltd. and Korean Patent Application No. 2002-77323 assigned to Electronics and Telecommunications Research Institute (ETRI) disclose other types of UWB system antennas. These patent applications disclose a planar antenna or an inverse L-shaped antenna having a relatively small and wideband characteristic.
IEEE 802.11a and HYPERLAN/2 regulating the standards regarding wireless LAN regulates that a frequency band of 5.15 to 5.825 GHz (Unlicensed National Information Infrastructure (UNII) frequency band), which is included in a frequency band available to the UWB, be used in the wireless LAN. These standards may cause interference with the UWB system in the UNII band since a high-power signal is used. Accordingly, in the UWB system, the use of the UNII frequency band overlapped with that of the wireless LAN is limited.
However, the antennas disclosed in the above U.S. Patents and Korean Patent Applications have only the UWB characteristic, but do not have a band-stop characteristic at a frequency band whose use is limited. Therefore, in order for these antennas to be actually applied, it is required that a band-stop filter having a high quality factor against a frequency band overlapped with that of the wireless LAN be additionally used. However, to add the band-stop filter not only increases the cost, but also limit the miniaturization and light weight of an equipment. The addition of the band-stop filter also causes the distortion of a pulse in the UWB system using a very short pulse, resulting in a degraded performance.